A seal support system typically comprises a vessel or tank which generally contains a volume of fluid. The vessel is piped to a sealing device on a pump, mixer or item of rotating equipment. Generally a return pipe leads back to the vessel from the sealing device, hence closing the “loop”. This allows the fluid, contained in the vessel, to enter and exit the sealing device. Typically such a seal support system is employed with a mechanical seal with two or more sets of seal faces, more commonly referred to as a double or dual seal.
The fluid within the vessel is generally chosen so that it lubricates and cools the components within the sealing device, whilst being compatible with the process fluid. The industry term for the fluid contained within the vessel is barrier or buffer fluid.
It is not uncommon for the seal support system to have other items of equipment, sited around the vessel, to permit pressure to be applied to the barrier/buffer fluid, or to allow additional cooling or fluid circulation around the seal.
Generally, the sealing device sited on the item of rotating equipment is a mechanical seal comprising a rotating member, which is secured to a shaft, and a stationary member which is secured to a housing.
The interface, between the rotating member and the stationary member, on the mechanical seal, prevents the processed product from escaping.
The majority of mechanical seals have a fluid film acting between the two sliding seal faces. This fluid film lubricates the seal faces. The wider the fluid film, the more the mechanical seal is liable to leak. Therefore, over time some fluid loss from the vessel is to be expected.
Furthermore, mechanical seals are often subjected to process upsets. Pressure surges and sudden rises in fluid temperature can result in barrier/buffer fluid loss.
Yet further loss of fluid from the vessel may result from evaporation.
Conventional dual seals should have the barrier fluid set at a higher pressure than the process pressure as this ensures clean barrier fluid lubricates the sliding seal faces instead of contaminated process fluid.
Unfortunately, some items of rotating equipment, for instance, equipment used in batch processes, can be subject to fluctuating process pressures in the seal chamber. It is not uncommon for the process pressure, in a typical application, to change from, say, 2 bar (30 psi) to 12 bar (175 psi). Given that this pressure fluctuation is a primary reason why the mechanical seal will fail, operators set the barrier fluid higher than the maximum process fluid pressure. From the above example, the seal would be set at 14 bar (205 psi). This means that when the process pressure is at 2 bar (30 psi) the seal barrier fluid is at 14 bar (205 psi) creating a massive closing force on the two sets of seal faces. This closing force increases the heat generated by the seal which in turn breaks down the fluid film between the seal faces resulting in premature seal failure.
In order to provide a solution to this problem, the pressure in the equipment seal chamber is tracked to create a reference pressure. This reference pressure is then connected to a pressure source, typically nitrogen which is connected to the seal support vessel. The nitrogen then pressurises the barrier fluid to ensure it is greater then the reference pressure. This system ensures that the barrier fluid maintains a constant pressure above the process pressure, irrespective of the process pressure fluctuations.
The problem arises when the process application requires a barrier fluid media which is not compatible with the pressure gas/nitrogen. An example of this is an oil barrier fluid where the nitrogen acts to aerate the oil, effectively injecting small gas bubbles into the oil. If these gas bubbles work their way to the mechanical seal faces, they will cause the seal faces to dry run, resulting in premature seal failure.
Another issue is that the seal support systems can operate in a variety of ambient temperature climates which fluctuate due to geography and time of day in addition to the process fluid temperature fluctuations. If the barrier temperature changes between night and day, as found, for instance, in applications in Saudi Arabia in the Middle East, this changes the barrier fluid pressure which results in changes in the system pressure differential constant between the barrier and process fluid.
A further issue in conventional systems is that the process fluid reference pressure, taken from the seal chamber, is directly connected to the accumulator. This can lead to chemical compatibility issues between the process fluid and the accumulator parts as well as seizing the accumulator when used with viscous process media.